In the manufacture of stretchable webs, such as creped tissues or paper towels, the control of basis weight is important from the standpoint of quality control and process economics. In the tissue industry, the basis weight of a web is generally expressed in terms of grams per square meter. Typically, tissue basis weights may range from about 10 to about 35 g/m.sup.2 and paper towels may range from about 20 to about 70 g/m.sup.2. Current industrial practice in the tissue industry is to control basis weight by measuring the basis weight of the tissue web at a point between the Yankee dryer and the reel and adjusting the mass flow rate of the stock system as needed based on that measurement. In so doing, the speed of the web is generally assumed to be linearly related to the speed of the reel. The speed of the web is also assumed to be constant at the selected location of the basis weight sensor with respect to other possible variables. Since the web speeds at the Yankee and the reel are known and fixed by their respective drive systems, the basis weight of the web at the reel can be calculated by using the basis weight of the web measured between the Yankee and the reel and an estimated web speed.
The problem with this approach is that the speed of the web between the Yankee dryer and the reel is not constant and is not linearly related to the speed of the reel for stretchable webs such as tissue. This is shown in FIG. 1, which is a plot of the tissue web velocity between the Yankee and the reel. In fact, the web speed can vary due to many factors which are not taken into account by current control systems, such as moisture content, formation, crepe uniformity, furnish, etc. as is shown in FIG. 2. As a result, the control system may adjust the stock flow when adjustment is not needed. Therefore there is a need for an improved method of controlling the basis weight of stretchable webs.